1. Field of the Invention
This invention relates to a novel semiconductor laser which is effective to control the transverse mode of laser oscillation.
2. Description of the Prior Art
Conventional semiconductor lasers are classified into two groups based on the structure of the active layer. One class consists of semiconductors lasers with a plane active layer and the of semiconductor lasers with a crescent active layer.
Semiconductor lasers with a plane active layer are inferior in that the oscillation threshold current is at a high level and the differential quantum efficiency is low, although they are excellent in that they attain laser oscillation in a fundamental transverse mode. On the contrary, semiconductor lasers with a cresent active layer are inferior in that they tend to produce laser oscillation in a high-order transverse mode, although they are excellent in that the oscillation threshold current is at an extremely low level and the differential quantum efficiency is high.
FIG. 1(a) shows a VSIS (V-shaped channel substrate inner stripe) semiconductor laser with a plane active layer, as an example of the above-mentioned semiconductor laser with a plane active layer, which comprises a p-GaAs substrate 111, an n-GaAs current blocking layer 112, a p-GaAlAs cladding layer 113, an n-GaAlAs plane active layer 114, an n-GaAlAs cladding layer 115, an n-GaAs cap layer 116, a top electrode 117 and a bottom electrode 118. The VSIS laser with a plane active layer, which is disclosed in TGED 81-42, 31 (1981-July) IECE Japan, attains laser oscillation in a stable fundamental transverse mode up to an optical output power of 20 mW, but the oscillation threshold current level is as high as 40-60 mA and the differential quantum efficiency is as low as 15%.
FIG. 1(b) shows a VSIS semiconductor laser with a crescent active layer, as an example of the above-mentioned semiconductor laser with a crescent active layer, which comprises a p-GaAs substrate 111, an n-GaAs current blocking layer 112, a p-GaAlAs cladding layer 113, an n-GaAlAs crescent active layer 214, an n-GaAlAs cladding layer 115, an n-GaAs cap layer 116, a top electrode 117 and a bottom electrode 118. The VSIS laser with a crescent active layer, which is also disclosed in TGED 81-41, 31 (1981-July) IECE Japan, although the oscillation threshold current level is as low as 20 mA and the differential quantum efficiency is as high as 25%, tends to produce laser oscillation in a high-order transverse mode even at a low output power (e.g., 1-3 mV), because the prevention of light from transversely expanding is dependent upon the difference .DELTA.N in the effective refraction index which is based on the difference .DELTA.d in the layer thickness resulting from the concaved shape of the active layer. The difference .DELTA.N is 1.times.10.sup.-2 or more which is exceedingly great. Therefore, if the difference .DELTA.d in the thickness of the crescent active layer is small and the difference .DELTA.N in the effective refraction is as small as around 2.times.10.sup.-3, laser oscillation in a fundamental transverse mode will be attained.
On the other hand, in the VSIS semiconductor laser with a plane active layer shown in FIG. 1(a), light goes through a waveguide depending upon both the difference .DELTA.N in the effective refraction index and the difference .DELTA..alpha. in the loss of the active layer 114 between the inside and the outside of the V-shaped channel 119, so that even though the difference .DELTA.N is maintained at a high level, light in a high-order transverse mode in the outside region of the V-shaped channel 119 is absorbed by the current blocking layer 112 thereby maintaining a fundamental transverse mode up to a high output power. However, as the difference .DELTA.N becomes great, the difference .DELTA..alpha. becomes great, e.g. 1000-2000 cm.sup.-1. When the difference .DELTA..alpha. is great, the light phase is delayed so that the beam waist tends to be positioned in the inside of the facets of the laser. Thus, conventional VSIS lasers have the drawbacks that beam waists in the direction parallel to the junction are at position of from 5 to 15 .mu.m from the facet, resulting in an astigmatism.
A window stripe semiconductor laser was proposed in Appl. Phys. Lett. May 15, 1979 P. 637, wherein the absorption of laser light around the facet is reduced to attain laser oscillation at a high output level without deterioration and/or damage to the facet. Japancese Patent Application No. 57-91636 proposes another window stripe semiconductor laser as shown in FIGS. 2(a) and 2(b), wherein an optical waveguide is formed in the window region of the above-mentioned window laser in Appl. Phys. Lett. to control both the beam waist and the high-order transverse mode thereby attaining laser oscillation in a fundamental transverse mode alone. This window laser comprises an n-GaAs current blocking layer 312 which cuts off current on a p-GaAs substrate 311. In the current blocking layer 312 and the GaAs substrate 311, a striped channel 319 having the width of Wc.sub.1 and a striped channel 419 having the width of Wc.sub.2 (Wc.sub.1 &gt;Wc.sub.2) are continuously formed. A p-GaAlAs cladding layer 313, a GaAs or a GaAlAs active layer 314, an n-GaAlAs cladding layer 315 and an n-GaAs cap layer 316 are successively disposed thereon. Electrodes 317 and 318 are disposed on the cap layer 316 and the GaAs substrate 311, respectively. The portion of the active layer 314 corresponding to the channel 319 has a concaved shape to form an optical waveguide having a width which is narrower than the width Wc.sub.1 of the channel 319. The portion of the active layer 314 corresponding to the channel 419 is plane shape to form an optical waveguide having substantially the same width as the width Wc.sub.2 of the channel 419, based on the fact that the effective refraction index becomes small due to light absorption by the n-GaAs current blocking layer 312 on both sides of the channel 419.
FIG. 2(a) shows the center portion of the window laser which is corresponds to a VSIS semiconductor laser with a crescent active layer shown in FIG. 1(b). FIG. 2(b) shows the vicinity of the facet of the windown laser which is correspondent to a VSIS semiconductor laser with a plane active layer shown in FIG. 1(a).
It has been found that the above-mentioned window VSIS semiconductor laser cannot be mass produced since lasers maintaining a fundamental transverse mode up to an optical output power of 20 mW or more at one of the facets thereof can only be obtained with a low yield. This is because of the difficulty of controling the curvature of the active layer 314 in the inside region of the laser. When the active layer is exceedingly concave, laser oscillation in a high-order transverse mode tends to be produced and even though the plane window region of the active layer 314 is prolonged in the facets of the laser, sufficient laser oscillation in a fundamental transverse mode cannot be attained. Moreover, the width Wc.sub.2 of the channel 419 of the window VSIS semiconductor laser is extremely narrow (e.g., approximately 4 .mu.m) and the thickness of the p-cladding layer 313 is then (e.g., approximately 0.1 .mu.m) so that light absorption by the current blocking layer 312 on both sides of the channel 419 is increased, resulting in astigmatism and/or a decrease in the differential quantum efficiency.